Protective coatings have been extensively use by the mechanical, electronic, and aerospace industries to impart erosion, abrasion, corrosion, and oxidation resistance to components and increase their useful life. Several techniques such as chemical vapor deposition, physical vapor deposition, electroplating, and similar methods have been developed and used for depositing protective coatings. Electroplating and similar techniques have been extensively used to deposit metal or mixtures of metals for providing corrosion resistance. Chemical vapor deposition (CVD) processes have been used to deposit metallic and ceramic coatings on various substrates to improve wear, corrosion, and oxidation resistance performance. CVD is particularly suitable for coating 3-dimensional and complex shaped parts because the deposition technique is indirect in nature, i.e., where the reaction gases are not in the "line-of-sight" with the substrates. Most of the CVD coatings, such as titanium nitride, titanium carbide, silicon nitride and silicon carbide, require use of high deposition temperatures (.gtoreq.900.degree. C.), thereby limiting their application to components made of temperature insensitive materials such as graphite, ceramics, and specialty alloys. Such coatings are, therefore, not suitable for many components made of ferrous and nonferrous metals and alloys because of degradation in their mechanical properties and distortion in shapes and sizes. Accordingly, there is a need to deposit protective metallic and ceramic coatings on components made of ferrous and nonferrous metals and alloys at low-temperatures without degrading their mechanical properties and distorting their shapes.
Several physical vapor deposition (PVD) techniques such as sputtering, reactive sputtering, arc evaporation have been developed and are currently being used to improve the wear and corrosion resistance of many metallic components. Although PVD techniques are being currently used commercially for many applications, their use is limited to a few coatings, e.g. titanium nitride, aluminum nitride, zirconium nitride, and similar materials, and to a few applications because of poor coating adhesion and defects in the microstructure. Therefore, there is a need to develop a low-temperature CVD process for depositing metallic and ceramic coatings on metallic components and improving their wear performance.
PECVD has been developed especially by the electronic industry to deposit various types of coatings on a variety of substrates. The process utilizes the energy of plasma to deposit protective coatings on substrates at low-temperatures. The plasma or glow discharge in PECVD is generated either by applying electric potential between two electrodes placed within the reactor under vacuum (called DC glow discharge) or by applying a radio frequency to one electrode while grounding the other under vacuum (called capacitively coupled RF glow discharge).
In the DC glow discharge process the negative and positive potentials are generally applied to the substrate and chamber wall, respectively. The substrate is generally heated by the bombardment of ions, thereby preventing independent control of temperature of the substrate.
In the RF glow discharge process an RF plasma field is generated by applying a radio frequency to one of the electrodes while grounding the other. The electrodes can be placed inside the reaction chamber. They can also be located outside the reaction chamber provided the chamber comprises a non-conductive material such as glass, quartz and the like. Because of the difficulty in generating a stable plasma by RF, most of the commercial reactors use parallel electrodes, which are placed inside the reaction chamber and are designed to coat planar substrates. The substrates to be coated are generally placed on the grounded electrode, which can be heated independent of the RF field. Furthermore, a DC bias can be applied to the substrates to control the reaction mechanism. The parallel plates in the RF plasma reactor are generally placed 2-3 cm. apart, making such a reactor unsuitable for uniformly coating 3-dimensional parts.
DC and RF plasma enhanced CVD reactors have been used to deposit a variety of coatings as taught in a number of references, examples of which are set for below. However, none of these references describe a method which is suitable for uniformly coating 3-dimensional objects.
U.S. Pat. Nos. 4,226,897 and 4,328,258 describe a DC-glow discharge method of forming undoped and heavily doped a-Si layer on a planar surface by using silane or a mixture of silane, phosphine and helium. The substrate is placed on a cathode and resistively heated to deposit the coating.
U.S. Pat. No. 4,330,182 also describes a DC-glow discharge method of forming undoped and doped a-Si layer on one side of a planar or cylindrical substrates by using either silane or a mixture of silane, phosphine and helium. The substrate is placed on a cathode heated resistively to deposit the coating.
U.S. Pat. No. 4,289,822 describes a process for depositing photoconductive amorphous (Si.sub.1-x C.sub.x).sub.1-y (H).sub.y materials on planar surfaces by reactive sputtering and electron beam evaporation technique. It is also disclosed that the material can be deposited by conventional DC or RF glow discharge using a gaseous mixture consisting of SiH.sub.4 and CH.sub.4 and pressure ranging from 0.1 to 5 Torr. The substrate temperature is controlled between 100.degree.-200.degree. C. to obtain good quality amorphous Si.sub.1-x C.sub.x (H) film.
U.S. Pat. No. 4,525,417 describes a process for co-depositing metallic element(s) and hard carbon coatings. The carbon and the metallic element, which was selected from a broad list of elements including silicon, are deposited by means of cathode sputtering in an inert gas atmosphere. The key feature of the process is that the metallic elements do not form stable carbides such as silicon carbide.
U.S. Pat. No. 4,532,150 discloses a process for depositing amorphous silicon carbide of the formula Si.sub.x C.sub.1-x onto planar substrates using a conventional parallel plate PECVD reactor. In additional to teaching the conventional parallel plate plasma generating reactor, this reference teaches a continuous process for coating articles in which an RF powered electrode in rod-like form surround a drum-like rotatable grounded electrode. A film-like substrate is passed over the grounded electrode.
U.S. Pat. No. 4,534,816 describes an RF-plasma etching and deposition process using a conventional parallel plate reactor. The critical features of the reactor include electrode parallelism at low inter-electrode spacings, efficient wafer-cooling, confinement of plasma over the wafer to minimize RF and plasma leaking away from the reactor zone, uniform gas distribution and pump-out, and minimum RF and gas flow disturbances around the edge of a wafer.
U.S. Pat. No. 4,792,378 describes a process for depositing material by an RF-plasma technique in a conventional parallel plate reactor. The key feature of the process is the use of a gas dispersion disc comprising a number of apertures to improve deposition uniformity. The process and reactor are only suitable for coating planar surfaces.
U.S. Pat. No. 4,810,622 describes a conventional parallel plate PECVD process for making a heterojunction structure. It involves depositing n-type a-SiC:H, undoped a-Si:H, p-type a-SiC:H, and undoped and n-type a-SiC:H layers on a substrate by PECVD using either a mixture of SiH.sub.4 and CH.sub.4 with PH.sub.3 or B.sub.2 H.sub.6 or a mixture of SiH.sub.4 and H.sub.2.
U.S. Pat. No. 4,870,245 describes a plasma enhanced thermal treatment apparatus for nitridiation of silicon-bearing substrates, e.g. semiconductor wafers. The plasma electrodes encircle the reactor's outer walls, which are made of non-conductive material such as quartz, for generating a plasma in the reaction zone. In one embodiment, both of the electrodes are in the form of interdigitated fingers which extend at least the full length of the reaction volume. This reactor design does not allow one to apply bias to the substrate which is important for controlling the composition and microstructure of the coating. The patented reactor is particularly suited for depositing coatings on a plurality of flat plates.
GB Patent Application No. 2,181,460 describes an apparatus and method for depositing material uniformly on a planar substrates by using a plasma enhanced chemical vapor deposition technique. The uniformity is improved by introducing gaseous mixture into the deposition chamber through a multiplicity of apertures. The process is suitable for coating planar surfaces.
Yoshihumi Suzaki et.al., "Atomic Structure of Amorphous Si.sub.1-x C.sub.x Films Prepared by R. F. Sputtering," Thin Solid Films, volume 173, pages 235-242 (1898) teach an RF planar magnetron sputtering process for depositing amorphous Si.sub.1-x C.sub.x (where 0.ltoreq.X.ltoreq.1). The carbon atoms were shown to occupy substitutional sites of the amorphous silicone network uniformly in the range of 0&lt;x.ltoreq.0.38. Clusters of threefold carbon and of fourfold silicon and carbon existed in the film in the range of 0.38&lt;x.ltoreq.1.0. The films were deposited on the substrate maintained at room temperature. The deposition rate was claimed to increase with increasing carbon content in the film.
Kenji Yamamoto et.al., "Physical Properties and Structure of Carbon-Rich a-SiC:H Films Prepared by R. F. Glow Discharge Decomposition," Thin Solid Films, volume 173, pages 253-262 (1989) presented physical properties of carbon rich amorphous SiC:H films. The films were deposited using pure SiH.sub.4, H.sub.2 and CH.sub.4 glow discharge at 1.5 Torr and 300.degree. C. substrate temperature in a conventional parallel plate plasma CVD reactor. The films were described to have a composition of Si.sub.0.37 C.sub.0.63.
D. A. Anderson and W. E. Spear, "Electrical and Optical Properties of Amorphous Silicone Carbide, Silicon Nitride and Germanium Carbide Prepared by the Glow Discharge Technique," Philos Mag, volume 35, page 1 (1977) presented properties of amorphous silicon carbide. The Si.sub.x C.sub.1-x films (where 0&lt;x&lt;1) were prepared using a mixture of the SiH.sub.4 and C.sub.2 H.sub.4 at 0.4 to 0.8 Torr and between 600.degree. to 800.degree. K. temperature in an inductively coupled glow discharge reactor.
Kazuyoshi Karosawa et.al., "Silicon Carbide Coating by the Plasma CVD Method", #RB 1117, pages 1-5 reported on the deposition of silicon carbide by plasma CVD method. A gaseous mixture of silane, methane and argon was deposited at 200.degree. C. in a conventional DC plasma discharge reactor using parallel circular flat plate electrodes which resulted in a uniform coating of stoichiometric SiC on a stainless steel substrate.
A. Raveh et.al., "Characteristics of Radio Frequency Silicon Carbide Films," J. Vac. Sci. Technol. A5 (5), volume 2836-2841, Sept./Oct. 1987 presented properties of SiC films produced by a conventional RF plasma process. The crystalline hexagonal .gamma.-SiC films were produced from a mixture of tetramethylsilane, hydrogen and argon in a low-pressure inductively coupled RF plasma reactor.
J. W. Zon, et al., "Deposition and Study of Hard Carbon Films", J. Appl. Phys., volume 65(10), pp. 3914-3917, May 1989, presented a process for depositing diamond-like carbon films on planar substrates by RF-plasma enhanced CVD using methane gas.
A. Bubenzer, et al., "RF-Plasma Deposited Amorphous Hydrogenated Hard Carbon Thin Films: Preparation, Properties, and Applications", J. Appl. Phys., volume 54(8), pp. 4590-4595, Aug. 1983, presented a process for depositing diamond-like carbon films on planar substrates by RF-plasma enhanced CVD using methane gas.
M. R. Hilton, et al., "Composition, Morphology and Mechanical Properties of Plasma-Assisted Chemically Vapor-Deposited TiN Films on M2 Tool Steel", Thin Solid Films, volume 139, pp. 247-260, 1986, presented a process for depositing hard TiN coatings on planar substrates by RF-plasma enhanced CVD using a mixture of titanium tetrachloride and ammonia.
The PECVD process and reactor designs of the prior art references are limited to coating single, flat surfaces, and are unsuitable for coating three-dimensional objects.